A semiconductor device such as LED or a power module has a structure in which a semiconductor element is bonded to the top of a circuit layer formed of a conductive material.
A power semiconductor element for high power control that is used for controlling wind power generation, an electric vehicle such as an electromobile, and the like generates a large amount of heat. Accordingly, as a substrate on which such an element is mounted, a power module substrate obtained by bonding a metal sheet as a circuit layer having excellent conductivity to one surface of a ceramic substrate formed of, for example, aluminum nitride (AlN) has been widely used in the related art. Furthermore, sometimes a metal sheet is bonded as a metal layer to the other surface of the ceramic substrate.
For example, a power module substrate described in PTL 1 has a structure in which a circuit layer is formed by bonding a Cu foil to one surface of a ceramic substrate. In the power module substrate, the Cu foil is disposed on one surface of the ceramic substrate through a Cu—Mg—Ti brazing filler material interposed therebetween, and a heat treatment is performed to bond the Cu foil.
Incidentally, in a case where the ceramic substrate and the Cu foil are bonded to each other through the Cu—Mg—Ti brazing filler material as disclosed in PTL 1, an intermetallic compound containing Cu, Mg, or Ti is formed in the vicinity of the ceramic substrate.
The intermetallic compound formed in the vicinity of the ceramic substrate is hard. Therefore, in a case where a thermal cycle is loaded on the power module substrate, a high thermal stress is caused in the ceramic substrate. Accordingly, unfortunately, the ceramic substrate easily cracks.
In addition, in a case where the hard intermetallic compound is formed in the vicinity of the ceramic substrate at the time of bonding the ceramic substrate and the circuit layer to each other, a bonding rate between the ceramic substrate and the circuit layer may be reduced, and the ceramic substrate and the circuit layer may not be able to be excellently bonded to each other.
Therefore, for example, PTLs 2 and 3 suggest a power module substrate in which a ceramic substrate and a copper foil which becomes a circuit layer are bonded to each other by using a Cu—P—Sn-based brazing filler material and a Ti material.
In the inventions described in PTLs 2 and 3, a Cu—Sn layer is formed on the ceramic substrate side, a metal compound layer containing Ti is formed on the Cu—Sn layer, and a hard intermetallic compound layer is not provided in the vicinity of the ceramic substrate. Therefore, the thermal stress caused in the ceramic substrate in a case where a thermal cycle is loaded can be reduced, and the occurrence of cracking in the ceramic substrate can be inhibited.